The present invention relates to mechanical fixation devices and hardware used in surgical applications to attach or anchor bone tissue or prosthetic devices to bone, or to secure pieces of bone together. Some examples of such devices are bone screws, bone pins, eye hooks and anchor assemblies. It is a common feature of these devices that they are fitted into a precisely drilled or reamed hole and they are intended to anchor firmly against one or more bone contacting surfaces. In particular, bone screws generally have extensive thread surfaces which cut or press against precut grooves in a hole drilled into a bone. Bone pins generally also similarly fit tightly against a hole wall, or have a bone engaging surface which allows them to fix at least one end of the pin. Fixation devices such as eye hooks may also be fastened to bone to allow suturing of soft tissue such as muscle, tendon or connective to a particular spot on the bone.
In general, these bone fixation devices have mechanical aspects analogous to those of the corresponding conventional fastening elements such as pins, screws, anchors and the like. In this regard, their functioning is dictated largely by a body or rod of high tensile, shear and bending strength, a plurality of screw threads with a sufficient lateral surface area for great load bearing ability, and in some instances self-tapping or wedging properties, and other mechanical structures which are substantially identical to those of the corresponding general purpose screw, pin or the like. Surface texture has figured relatively little, if at all, in the construction of bone fastener hardware.
In some areas of the bone prosthesis field, surface texture has been found to be quite important. Thus, when replacing acetabular joints or structural endings for major bones, e.g., artificial knees or hips, it has previously been realized that the surface texture of the metal prosthesis may be tailored to be more or less conducive to the ingrowth of bone material and thus the formation of a strong cement-free bond to existing bone. Textures for this purpose have been formed in a cast metal bone prosthesis, for example, by brazing on additional small surface features such as balls, wires, or screens which collectively define protrusions and indentations into and about which bone may grow to form a relatively strong and shear resistant coupling. In certain circumstances, the presence of suitably textured surfaces on such prostheses may allow the surgeon to dispense with cement or other bonding materials which otherwise had been generally required to form at least a temporary bond to the prosthesis during the process of bone healing. The scale of surface patterning which has been found useful in bone attachment extends from what might be described as a macro-roughness, with pores of approximately 100 microns and larger, to what might be described as a relatively jagged surface configuration with structures extending a millimeter or more outside the plane of the nominal prosthesis surface. As a rule, the utility of such features is related to their ability to support or encourage trabecular bone growth and the ultimate formation of a hard organic bond to the prosthesis.
On a different level, surface texturing such as knurling may be applied to symmetrical items such as bone pins to achieve an ultimate increase in the shear strength where they bond to a bone. Here, the relative feature size is again measured in fractions of a millimeter, and the bond-strengthening mechanism appears somewhat similar to that described for prosthetic bones in providing a network in which bone intergrowth may extend in a depth dimension to form a shear free interface. However, these designs for addressing the shear strength of the bone coupling layer address the relatively long term strength of the interface so formed; for such a design, the short term strength immediately following installation is not affected, and in fact, if installed without adhesive, the interface may be considerably more fragile due to the smaller direct and rigid contact area involved.
Also, items such as bone screws having a self-wedging taper may become loose quite quickly if they start to back off. Thus, care must always be exercised in fitting the bone fastener tightly enough without exerting such great force as to crack bone or risk subsequent loosening of the device.
Various structures have been developed for providing secure fixation, among which may be cited those of U.S. Pat. Nos. 2,381,050; 2,397,545; 2,490,364; 2,699,774; 3,678,925; 3,716,051; 3,782,374; 3,805,775; and 4,011,602; 4,355,428; 4,743,256; 4,745,913; 4,834,757 and 4,878,915. All of these structure address, to some extent, the provision of a mechanical gripping or biting mechanism that can be tightened sufficiently to provide a sure grip while limiting to some extent the magnitude of stresses created in the surrounding bone. This is generally done with protrusions that bite in localized regions, self-limiting wedging arrangements that distribute stress and provide only gradual expansion, porous or polymer mechanical contact faces, or combinations of these features. However, in general these fixation devices still rely on a screw-tightened wedging action, and often they provide a broad range of adjustment so that the correct feel or torque during installation to assure fixation without introducing destructive levels of strain is left to the mechanical skill, experience and judgment of the surgeon. Since the installation process itself necessarily generates debris that may create binding and provide a false sense of torquing into solid material, the success or longevity of installation may therefore involve some element of luck, even when performed by a highly experienced and meticulously careful surgeon.
Accordingly, there is a need for fixation devices that provide uniform and secure fixation characteristics.
There is also a need for bone fasteners having a mechanical design that offers improved tightening and holding ability, yet with less risk of creating high stresses in the bone to which they attach.